Utilization of fiber laser sources has increased in industrial and scientific applications over the last few years in metrology, imaging and material processing applications. Fiber-based laser systems are now well established for numerous applications, and are particularly well suited for high repetition rate applications at low to medium pulse power.
Nonlinear optical processes and media may be used to convert a fundamental output wavelength of a fiber laser to another wavelength where a fiber laser source is not available. A wavelength converted signal may be generated by harmonic conversion, sum or difference frequency mixing, parametric amplification, Raman shifting, self-phase modulation (SPM) and/or other suitable non-linear processes.
Utilizing multiple wavelengths can be beneficial. For example, in terahertz applications, one wavelength is used to generate THz radiation and the other wavelength is used to detect THz pulses. Similarly, in time gating pump and probe measurements the first wavelength is used to excite the object and the second wavelength is used as probe. In some laser processing applications, the first wavelength can be used to pre-process a target material and the second wavelength used to further process the target.
The following patents, published patent applications, and publications relate, at least in part, to fiber lasers and amplifiers, optical measurement techniques, and/or various arrangements for generating groups of laser pulses: U.S. Pat. Nos. 5,450,427; 5,818,630; 5,880,877. In addition, U.S. Pat. No. 5,361,268 discloses a switchable two-wavelength frequency converting laser system.
Various techniques can be used to switch between a pump and signal wavelength. As one example, switching laser output between the pump wavelength and the signal wavelength can be done by shifting a nonlinear crystal in and out of the pump laser beam, or by steering the pump laser beam on and off the crystal. However, this provides either the pump or signal as a single output, without continuous or high resolution control of either pump or signal power. Also, it is difficult to maintain the critical alignment. As one alternative, tuning the phase matching of the nonlinear crystal by changing temperature, incident angle, etc., can change the power of the signal light. However, the adjusting range is usually limited and the tuning may affect the wavelength. As another example, multiple optical paths can be utilized with optical modulators/deflectors to selectively direct pump radiation to the non-linear crystal or to the output, but such an arrangement can increase system complexity.
The phase matching condition in the nonlinear crystal is usually sensitive to polarization states of the input and output laser beams. For example, when a (ooe) type I phase matching is used, only the pump light, whose polarization is parallel to the ordinary axis of the nonlinear crystal, contributes to the useful nonlinear process. Therefore, the signal power can be controlled by changing polarization states of the pump source.
A nonlinear crystal allowing phase matching is usually a birefringent crystal. As a result, the polarization state of transmission light will be changed unless the initial polarization is parallel to an optical axis of the crystal. Under such conditions the output laser polarization may become elliptically polarized. The undefined elliptical polarization state may not be desirable for some applications where polarization is critical.
Wave plates, such as quarter wave plates and/or half wave plates may be used to correct the phase delay. However, simply adding wave plates into the beam path may not completely compensate the phase delay generated by the nonlinear crystal. The polarization states may vary whenever the energy distribution is changed, which requires an adjustment of the wave plates. Additionally, wave plates will affect polarization states of both pump and signal wavelengths if they are not spatially separated in advance. Separating the pump and signal light, as well as adjusting the wave plates, increases complexity of the laser system and its operation.